Pre-heat oven to 375*, line 2 cookie sheets with parchment paper and set aside.
Sift together the flour,baking powder, and salt onto a sheet of waxed paper. In a large bowl cream together the butter, shortening, and both sugars until light and creamy about 3 min. Add the buttermilk and beat until combined.
In a measuring cup combine the milk, baking soda and vinegar. Add to the batter along with the flour mixture and beat on low until well combined. Add the vanilla and beat on medium speed until completely combined.
Using a spoon or scoop drop 1 tablespoon of batter on to the cookie sheets and repeat, spacing them 2 inches apart.
Bake one sheet at a time for 8-10 minutes or until the edges begin to brown. remove from oven and let cool about 5 min. before transferring them to a rack.
Meanwhile in a medium bowl beat together the peanut butter and butter on low speed until smooth and creamy. Add the confectioner's sugar and salt, beat on low to incorperate.
Increase the speed to medium and beat until the filling is light and fluffy, about 4 minutes.
**NOTE** yes this is salty but it doesn't need to be you can leave out the salt.